Nutritional perspectives in female soccer: a scoping review

ABSTRACT Background The purpose of the review was to evaluate the literature exploring nutritional habits and practices in female soccer players. Methods The PRISMA-ScR Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews were followed. Searches of Web of Science, PubMed and Scopus databases were conducted for studies exploring the nutritional habits and practices of female soccer players. Results A total of 72 studies were included in the scoping review. Studies on female soccer players mainly focused on daily energy expenditure, daily energy and macronutrient intake and hydration status. A negative energy balance was consistent across studies, and the ingestion of CHO appears below the current recommendations. Female soccer players are predominately in negative energy balance, which may indicate that they are at risk of low energy availability. A high use of nutritional supplements is apparent in female soccer, whilst a large proportion of players commence training dehydrated. Conclusions The current findings have implications for practitioners relating to the planning, management, monitoring, and implementation of nutritional intake and training and competition schedules.


Background
The popularity and professionalism of female soccer has increased markedly in recent years across all levels of the game.Data from the Fédération Internationale de Football Association (FIFA) obtained between 2015 to 2019 also reported an increase in female soccer players in different countries.It is estimated that 13 million females are playing organized soccer, and 63,000 coaches are working with female teams [1].The rise in popularity has been accompanied with a 6-fold increase in published research papers [2].The majority of research in female soccer involves observational studies objectively quantifying the match demands in female soccer [3][4][5].However, there are fewer studies exploring the nutritional habits of female soccer players.Therefore, it is likely that most of what is currently known about and adopted in relation to female soccer is derived from observations or practices that have demonstrated efficacy in male soccer athletes.This could have detrimental implications for female soccer given that females possess different physiological profiles [6,7] that are likely to influence dietary needs between sexes.
International female soccer players competing in European leagues complete approximately 10 km total distance, 2.5 km high-speed distance (> 14.4 km.h −1 ) [3], 174 accelerations and 20 sprints (168 m) during official matches [8].Training and competition demands are on an upwards rise in elite female soccer players.Consequently, this will lead to greater energy demands and the need for dietary modification to meet such demands.However, since there is a lack of empirical projects exploring the current landscape in female soccer, energy demands and nutritional habits are not fully understood.Players that fail to meet daily energetic and nutritional recommendations are at risk of low energy availability, commonly defined as < 30 kcal −1.kg FFM −1.day.Such low energy intake is related to adverse health effects, such as compromised bone health, irregular menstruation, and/or reduced immune functioning [9].Therefore, it is key that a balanced appraisal of the literature is carried out in relation to the nutritional habits of female soccer players.Energy availability and energy balance are similar but distinct terms, with the limitations of these concepts widely debated in the literature [10].Energy availability represents the energy to sustain physiological functions after the cost of exercise has been met, and is calculated as energy intake minus exercise energy expenditure, expressed relative to fat-free mass [11,12].Energy balance occurs when energy intake equals energy expenditure and is affected by numerous components of energy expenditure, including, exercise energy expenditure, resting metabolic rate, and dietary -and cold-induced thermogenesis, with this concept calculated via the subtraction of energy expenditure from energy input [13].Previous studies have demonstrated that daily energy expenditure remains higher than daily energy intake in elite English [14] and Dutch players [15], indicative of a negative energy balance.Evidence suggests that soccer players tend not to meet the general recommendations for CHO intake in male soccer [16].Given the negative implications of being in negative energy balance, particularly in females [12], characterizing the nutritional practices in this population remains a key priority.
Given the increasing popularity of female soccer, it is perhaps surprising that reviews summarizing the nutritional practices are scarce.A recent narrative review analyzed daily energy and macronutrient intake of seven studies using elite female soccer athletes [17].However, there are no scoping reviews that have undertaken a preliminary assessment of the potential size and scope of the available research relating to energy and nutritional behaviors, dietary habits and supplementation in female soccer players.Furthermore, the heterogeneity of player age (16 − 41 years), stature (1.48 − 1.87 m) and mass (46 − 88 kg) among top-level players indicates that studies of a range of competitive groups and youth athletes should also be considered [18].Therefore, this scoping review investigated nutritional habits and practices, including nutritional periodization, micronutrient intake and supplementation use, among female soccer players.

Methods
The present scoping review followed the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) Checklist [19].The protocol was developed by the research team and registered on the Open Science Framework at d oi:10.17605/OSF.IO/2YU3D.

Eligibility criteria
The inclusion criteria of the current scoping review were defined based on the type of participants, concept, context and types of evidence sources [20].Studies that involved female youth or adult populations participating in competitive soccer were considered for the present review.The concept explored studies that investigated nutritional issues, dietary habits and supplementation of female soccer players.Types of evidence contained original research or dissertations written in English, Portuguese or Spanish.Reviews, case studies and letters to the editor were not analyzed.There were no defined restrictions regarding the year of publication or geographical location.

Information sources and search
The search strategy was divided into three different phases [21].A preliminary search relating to nutrition in female soccer was conducted using PubMed.The titles and abstracts were originally screened and selected by two independent authors (DVM/HS).Three databases were consulted (i.e.Web of Science all databases, PubMed and Scopus) on 20 November 2022 and updated on 2 April 2024.The search results were exported into a reference manager software (EndNote X9; Thomson Reuters©, New York, NY, USA).Duplicates were automatically removed and manually checked to ensure that duplicates were removed.Manual searches of each study of the bibliography extracted from the first search were carried out to identify additional studies.Searches included the following keywords: football OR soccer AND female OR women AND nutrition* OR nutritional status OR nutritional intake OR intake OR ingestion OR nutrition habit* OR diet* OR energy expenditure OR energy intake OR dietary intake OR dietary status OR dietary supplement OR ergogenic aid* OR macronutrient* OR micronutrient*.

Selection of sources of evidence
The references were exported to EndNote which were assessed by two authors (DVM/AR).Records that did not follow the eligibility criteria in terms of population, context, content and types of evidence were initially excluded according to the titles and abstracts.Fulltexts were consulted to identify studies that met the inclusion criteria.The screening of references and citations was also performed by two independent observers (DVM/AR) and disagreements between authors were solved by a third independent researcher (HS).Manuscripts were omitted if they violated any of the following criteria: involved male soccer players, included school or American, Australian or Gaelic football, contained no relevant data about nutritional aspects.

Data charting process, data items and synthesis
A data-charting sheet created on Microsoft Excel summarized the following parameters: authors, year of publication, journal, country, aim, sample characteristics (age, stature, mass, body composition, main results, limitations, conclusions and practical applications).Some of the information extracted from the manuscripts was not presented in the results section, but was retrieved to gain a comprehensive depiction of the study.Two authors (DVM/AR) independently extracted the relevant details, and the final information of studies included in the present review was confirmed by consensus.Study characteristics were summarized in tables according to the following topics: daily energy (intake and expenditure), fluid balance, supplementation, nutritional knowledge, other topics.

Sources of evidence
During the initial searches, 52 studies were identified as eligible for the current review, with two additional articles located by the authors during manual searches.The updated searches identified 18 manuscripts that met the eligibility criteria.As a result, 72 papers were included in the scoping review (Figure 1).

Characteristics of sources of evidence
As shown in Table 1, female soccer players from the United States were included in 17 studies (24%), eight studies involved Polish players (11%), and seven studies contained soccer participants from the United Kingdom (~10%).Studies were mainly developed using adult participants (69%), while 18% were conducted with female youth participants (mean age < 18 years).Excluding the youth populations, 40% of studies involved participants competing at national level, 25% were university players and 13 studies incorporated elite players (22%).There was no intervention effect on performance.Regarding the biomarkers analyzed it was noted an increase of interleucine 10 when Greek yogurt was ingested.However, negligible changes were verified for inter leucine 6, TNF α and creatine-kinase.In brief, the positive impact of Greek yogurt on performance and recovery was not evident.
Morehen et al. [ e. athletes designed own menus with an exchanged list).Dietary intake using 7-day food records, nutritional knowledge using the General Nutrition Knowledge Questionnaire.Skinfold thickness was measured, running speed and jumping performance were evaluated and blood samples were collected.To improve nutritional the exchange diet seems to be a good strategy.Control diet may help to normalize hemoglobin concentration and prevent anemia.Apparently, skinfold thickness was reduced in both type of diets.Improvements were not noted in soccer-specific skills.
Sebastiá-Rico et al. specific gravity measured using a digital refractometer, urine color.Weight loss, sweat rate, fluid intake was higher during the summer in comparison to winter.Fluid balance was affected by sex, climatic conditions and competitive level (values tend to be higher in male soccer players).

Results of individual sources of evidence
Allowing for variation in methods used for daily energy intake and the level of participants, the caloric intake (kcal.day−1 ) was widely variable across studies of adult and youth female soccer players (Table 2).In adult players, daily energy intake ranged from 1476 kcal in professional Polish soccer players [55] to 3050 kcal in Spanish Super League players [33].An average intake of ~2060 kcal was obtained for 30 studies that included adult female soccer players.The energy intake varied from 1622 kcal.day−1 in youth Canadian players [74] to 3122 kcal.day−1 in youth Iranian players [49].Different methods were used to obtain daily energy expenditure.One study measured energy expenditure by doubly labeled water; with energy expenditure identified as 2963 kcal.day−1 [75].Three studies characterized energy intake at different phases within the season in adult participants [26,37,40].The daily energy intake was substantially higher in pre-season (2290 kcal.day−1 ) versus post-season (1865 kcal.day−1 ) in US soccer players [26].Comparable findings were noted in another study conducted with adult American players [37], with the intake during pre-season (2794 kcal.day−1 ) higher than inseason (2208 kcal.day−1 ) and off-season (2161 kcal.day−1 ).The energetic intake of Brazilian players was examined on training day, match day and post-match day [40].The intake was comparable on the training day (2386 kcal) and post-match day (2282 kcal), but it was significantly lower on match day (1940 kcal) [30].
Twelve studies reported the statistics (mean ± standard deviation) of daily energy intake and expenditure (Figure 2).In studies of adults [22,30,44,55,57,58,73,85,93] and youth players [32,52,53], raw mean differences between energy intake and total daily energy expenditure confirmed negative energy balance of -441 kcal.day−1 (95% CI: -636 kcal.day−1 to -247 kcal.day −1 ).Some studies focused on the relationship between daily energy intake, exercise energy expenditure and body composition on energy availability [37,39,60,67].Table 3 summarizes macronutrient intake in female youth and adult soccer players.Among adult players, relative CHO intake varied between 3.3 g.kg −1 to 7.0 g.kg −1 , and protein ingestion ranged from 0.96 g.kg −1 to 2.0 g.kg −1 .Among youth players, the values of protein ingestion were comparable in three different studies while mean values of CHO intake ranged from 5.0 g.kg −1 to 7.9 g.kg −1 .Fourteen studies reported the relative fat intake, and mean values fluctuated from between 1.0 to 2.0 g.kg −1 .
Five studies focused on hydration status [35,43,48,63,65].Overall, a large proportion of players tended to start soccer practice dehydrated.The percentage of players hypo hydrated ranged from 44─78% in 18 players from the under-19 Belgium team [48].In the remaining studies, the prevalence of hypohydration before soccer sessions ranged from 45─54%.SD (standard deviation); a intake was detailed for pre-season and off-season; b intake was detailed for match, post-match and training days; c two groups were analyzed: supplementation and control; d intake was detailed for pre-season, inseason and, off-season; e authors did not present the SD.

Discussion
The present scoping review aimed to summarize existing scientific evidence investigating nutritional habits and practices in female soccer players.The main findings emerging in the current review include: (1) daily energy intake is widely variable across studies, (2) although different methods have been used to assess daily energy expenditure, a negative energy balance was consistent across studies, (3) the recommendations of CHO are rarely met by female soccer players while protein intake seems to be within the recommendations, (4) a significant percentage of female players ingested supplements, and (5) energy intake is the highest during preseason versus other phases of the year.

Energy balance and energy availability
The concept of energy balance reflects the difference between total daily energy intake and energy intake expenditure [11].Energy deficit was found in the present review, which may have a negative impact on growth, health and performance [94].Negative energy balance was highlighted in specific recommendations developed for youth athletes in such a way as to avoid long periods of energy restriction [95].Many studies included the combination of youth and senior soccer players [23,45,55,71].For instance, the age of Polish professional players ranged from 13 to 31 years old [55].Given the differences in metabolic profiles between adults and youth populations [96], determining energy balance to formulate accurate nutritional plans remains a key challenge for nutritionists.Therefore, although using accurate techniques to estimate daily energy outputs of female soccer players may be logistically challenging, it is key that attempts are made to provide individualized targeted nutritional plans.Moreover, the studies included in the present scoping review were cross-sectional since they only described the total energy expenditure and energy intake of female players.The total energy expenditure represents the sum of resting energy expenditure, thermic effect of food, and exercise energy expenditure.Nutritionists often use equations to predict resting energy expenditure [97] based on a fixed physical activity coefficient multiplied by resting energy expenditure [98].An additional challenge is the assessment of energy intake since the most common methods (i.e.weighted food records or food diaries) tend to underestimate the actual dietary intake [99].Consequently, the mean values of total energy expenditure and energy intake reported are questionable, considering the error associated with these methodologies.
Although few studies in the present review compared changes in energy balance and body mass; one study reported a negative energy balance of -825 kcal.day-1across four days of assessment, but with negligible variations in body mass [75].Therefore, this supports that energy balance should be measured over an extended period in conjunction with changes in body mass to be able to provide robust inferences that a negative energy balance has occurred.The principle of energy balance based on variation of body mass compartments (i.e.fat mass and fat-free) mass was recently described as useful to determine energy intake in the context of energy availability [10].Energy availability is defined as the difference between energy intake and exercise energy expenditure once normalized for fat-free mass.This corresponds to the amount of dietary energy available for normal physiological functioning [100].
-1 [75].Different methodologies could explain the inconsistent findings to examine daily energy intake, exercise energy expenditure and body composition which define the concept of energy availability [101].The evaluation of dietary intake was reported using multiple day food diaries [37,39,52,61,68].Exercise energy expenditure was estimated based on exercise diaries [52,68], wearable devices [60,61] or based on total energy expenditure, resting energy expenditure and the thermic effect of food [71].Fat-free mass and lean soft tissue were obtained by dual-energy X-ray absorptiometry [37,39,71,75], air-displacement plethysmography [61] or bioimpedance [52,60].The pre-assessment conditions of air-displacement plethysmography and bioimpedance were not described in most of the studies.Unstandardized procedures (i.e.no control before the measurement) of these methodologies are a central factor in creating bias in body composition assessment [102].The breakfast ingestion and daily activities were also reported as sources of error in measuring lean soft tissue in dual-energy X-ray absorptiometry scanning [103].Indeed, variation of methodologies used to obtain fat-free mass are often ignored and may need to be corrected for accurate interpretations of energy availability [101].Fluctuations in mean values of energy availability tend to occur across the season, which is affected by variations in exercise energy expenditure and daily energy intake during pre-season, inseason and off-season [37,101].Meanwhile, acute periods of extreme energy deficits have no negative effects on performance if the players maintain an average energy availability within the adequate limits [101].Nevertheless, players that were classified with low energy availability tended to exhibit a lower relative CHO intake [39,52,68].Considering variations in training load and competitive schedules, maintaining an overall weekly energy availability within an acceptable interval and meeting the macronutrient recommendations is integral [101].

Soccer nutritional recommendations
Soccer-specific nutritional recommendations have not always differentiated between male and female athletes [94].Based on training period and external load (duration, training distance, high-speed running), ranges of CHO have been provided for pre-season (4-8 g.kg −1 ), in-season with one match per week (3-8 g.kg −1 ), in-season with congested fixtures periods (6-8 g.kg −1 ) and off-season training (< 4 g.kg −1 ).Studies included in the present review showed mean values were nearer to the lower limit of CHO recommendations [55][56][57]61,68,75].Female soccer players recognized the importance of fueling initial training but did not identify CHO as the primary energy source during exercise.Players tend to associate CHO intake with gains in fat mass and a negative appreciation of body shape [78].One study found comparable values of mean relative CHO intake in preseason, in-season and off-season [40].Therefore, nutritionists should explain the importance of CHO intake to athletes and provide specific strategies to ensure CHO periodization considering training load and match demands [16].The recommendations for CHO also focused on the hours before the match, during the match and post-match [94].Two investigations included in the present review examined the importance of CHO ingestion before [70] and during the match [46].Negligible differences in performance, perceptual changes, and physiological responses were found between high CHO and mixed macronutrient meals four hours before competition [70].A study examined the effects of carbohydrate mouth rinsing (solution with 6% maltodextrin) in short-maximal outputs among 11 female soccer players [46].Comparing CHO mouth rinsing and placebo trials, no differences in jump and velocity protocols were noted between conditions.Carbohydrate mouth rinsing is recommended for intermittent exercise given its benefits to sprint performance and purported lack of gastrointestinal symptoms [104,105].However, given that CHO studies are often exclusive to male players, the form, quantity and timing of CHO in female soccer players require further attention.
To promote muscle skeletal adaptations and recovery, a protein intake of 1.6 to 2.2 g.kg −1 is recommended [94].It has also been suggested that daily protein distribution (i.e.20-30 g per meal) and quality are also crucial to optimize body composition [106].In the present review, the mean values of daily protein ingestion were within the recommended values, but contrary to the literature on male soccer players, no studies have examined the protein intake throughout the day [107,108].Among 14 adult Dutch elite male players, a skewed distribution of absolute protein intake across the day was noted, with significantly higher ingestions observed at breakfast, lunch and dinner compared to morning, afternoon and night snacks [107].In youth female sports participants, protein intake recommendations tend to increase 2.3 g.day −1 especially during the maximal growth spurt [95].In the three studies that included adolescent female players [32,52,53], the protein intake was below recommendations.This point is critical if the energy requirements are not met, potentially reducing protein availability for its primary functions of repair and regeneration [95].
Specific nutritional recommendations for female athletes considered the impact of the menstrual cycle [18].It was suggested that exercise performance and muscle glycogen might be reduced during the early follicular phase (low levels of estrogen and progesterone) in comparison to the other phases of the menstrual cycle, with recommendations of 6-8 g.kg-1.day-1during this phase [6,109].However, studies on female soccer players have revealed inconsistent findings relating to the decrements in performance during the follicular phase and luteal phases [110][111][112].Nine elite players covered 2883 m during the Yo-Yo intermittent recovery test during the mid-luteal phase (high levels of estrogen and progesterone), while players covered 3288 m during the follicular phase [111].However, in a separate investigation, distance covered at very high intensities (16.69 km.h −1 to 19.94 km.h −1 ) tended to be higher in the luteal phase in contrast with the follicular phase [110].Additionally, estrogen has been associated with anabolic (i.e.glucose uptake in the muscle and recovery) and progesterone on catabolic functions [113].As such, nutritional guidelines for female soccer players should be carefully considered alongside the hormonal fluctuations associated with the menstrual cycle.Whilst specific nutritional recommendations for female athletes considering the influence of menstrual cycle require future research, practitioners may be advised to consider dietary modifications based on individual requirements and experiences.
Dehydration was frequently observed preceding soccer training or competition.Although slow rehydration is recommended by consuming 10 m l. kg −1 water 2-4 h prior to exercise [114], it is recommended that individualized interventions should be undertaken to examine hydration as well as personalized recommendations to attain hydration status before soccer training and competition [69].However, the studies included did not use consistent methods, nor was care taken to standardized procedures across testing days.Therefore, further research is needed on hydration in female soccer players before hydration guidelines can be provided.

Limitations
A potential limitation of the studies included in the present scoping review is the heterogeneity of the methods used to quantify energy intake and energy expenditure.Food diaries often under-reported energy intake [115], while only one study examined energy expenditure using doubly-labeled water method [75].The comparison of reference methods to evaluate total energy intake (i.e.food diaries, 24-hours recall) was contrasted with total energy expenditure measured by doubly labeled water in a metaanalysis that included 11 studies [99].The overall energy balance in all studies was negative (mean value: 700 kcal.day−1 ; 19%) and differences between total energy intake and energy expenditure tend to increase with higher total energy expenditure.This systematic bias could be explained by general (e.g.body size, image, motivation, social expectations, the environment of assessment) and sport-specific factors (e.g.energy needs, frequency of meals, knowledge of portion sizes, commercial foods, supplements) that affect dietary intake [99,116].Another possible limitation of the studies found in the present scoping review is the reporting of 24 hours of energy balance.This theory does not consider the endocrine responses associated with real-time changes in energy balance [117].Consequently, continuous monitoring of energy balance measured in smaller intervals seems more appropriate since time spent in energy deficit was negatively associated with cortisol values in male and female endurance athletes [118,119].

Conclusion
The current study provides a broad overview of the nutritional practices of female soccer players.Female soccer players are predominately reported to be in negative energy balance, which may indicate that energy intake should be prioritized, with particular focus on CHO intake.Carbohydrate periodization has been largely examined in male soccer players, with few studies in females.The adjustment and planning of macronutrient intake among females needs to consider the periodization of the training load and match schedule.Nutritional intake, energy expenditure and body composition should be frequently assessed, given the risk of low energy availability in female players.Future studies should consider consistent methodological approaches to the measurement and reporting of nutritional intake.Additionally, more intervention-based studies would be a valuable addition to the literature as much of the current research is descriptive in nature.An emerging topic that requires further investigation is the impact of the menstrual cycle on the relationship between soccer training and competition and nutritional guidelines.Nutritional practices are a key avenue of research that must be explored to ensure that performance and health are optimized in female soccer.

Figure 1 .
Figure 1.Flowchart of the review process.

Figure 2 .
Figure 2. Forest plot of the differences between energy intake and energy expenditure.

Table 1 .
Characteristics and main conclusions of the studies included in the current review.

Table 1 .
(Continued).The daily EI was 2386 kcal, 1940 kcal, 2282 kcal on training days, match day and post-match.When the values were expressed per kg of body mass, the respective values were 45.3, 34.2 ad 41.8.Relative intake of CHO was 5.9 g.

Table 2 .
Studies that reported total energy intake in adult and youth female soccer players.

Table 3 .
Macronutrients intake in female soccer players.pre-season; b off-season; c training; d match-day; e post-match; f pre-season; g in-season; h post-season; i rest day. a